I was thinking about how this calculation reminded me of all those exams in school where you had to work out acceleration or current or something and you knew when you had the wrong answer because the correct answer always came out magically to some nice round number. In this case the 241 is an obviously wrong answer because it isn't a number we've seen before, whereas 235.4 is a fundamental constant that we all recognise and which significantly gives something very close to £1.32 for the petrol price.

Except that reminded me of another reason why 235.4 is completely meaningless. Never mind whether or not you drive to match the NEDC expectations, there's a hidden fudge factor in there. The problem is the official figures are calculated in l/100km and are mostly rounded to one decimal place. That's fine if you have a car doing 6.0 or 6.1l/100km, that's a 2% error, but the difference between 1.1 and 1.2l/100km is nearly 10% and by the time you're down to 0.5 or 0.6 you've got a possible 20% error.

Before I bought my Volt I went through the calculations from the Ampera blog and couldn't match the 235.4 until I realised you have to round the earlier result before doing the conversion to imperial. After converting to mpg they then compound things by introducing more apparently significant digits: in 235.4 the 3 is actually wrong never mind the 5.4.

Volvo look to have partly realised this is wrong as their V60 mpg figure 155.2mpg equates to 1.82l/100km, so they must have kept the second decimal place, but they still have spurious digits in the imperial figure.

Here is how you calculate the 235.4 for an Ampera:

Electric range (RE) = 83km

Consumption in mode A (CA) = 0

Consumption in mode B (CB) = 5.0 l/100km

Weight consumption (CW) = re/(re+25)*ca + 25/(re+25)*cb = 1.15(740) l/100km

Rounded to 1dp = 1.2 l/100km

Convert to imperial = 235.4 mpg

Without that all important rounding step the number would have been 244±3mpg (assuming CB is ±0.05). Given that in practice the range is usually in the range 48 to 81km (so 83km is really an upper limit) a more realistic official figure would have been 244mpg with error bars +3/-80mpg.